Lead brass can be easily machined to parts with various shapes due to their excellent performances in cold and hot workability, cutting performance and self-lubricating. Lead brass have been always recognized as an important basic metallic material and have been widely used in civilian water supply systems, electricity and the field of automotive and machinery manufacturing. Because of its wide use, large numbers of lead brass parts were abandoned, where only a few were recycled, while many small parts were abandoned. When coming in contact with the soil, lead in abandoned lead brass would enter the soil under long-term effect of rainwater and atmosphere and contaminate soil and water. When abandoned lead brass was burned as garbage, the lead vapor would enter atmosphere and greatly harm human health, so the application of the lead brass was being tightly restricted. Lead is preferred to appear as lead micro particles of simple substance on grain boundaries, neither lead-copper solid solution alloy, nor lead-copper intermetallics. In drinking water, under the effect of impurities and ions etc, lead in the lead-copper alloy will be separated out as the form ions and lead to contamination. The existing lead-copper alloy is difficult to meet the requirements of environmental laws. In order to decrease the harmful effects of lead, the corrosion mechanism of brass in drinking water and the effect on corrosion mechanism of brass when adding elements were systematically studied, and a variety of measures were taken. For example, on one hand, a small amount of tin, nickel or other annoying elements were added to improve the corrosion resistance of lead brass; on the other hand, chromium or other corrosion-resistant metals were covered on the lead-free surface of lead brass which can be obtained by removing the soluble lead from the lead brass with a certain thickness by dissolving. There is no way to eliminate the harmful effects caused by lead because of its existence in basal lead brass. The lead brass whose cutting performance is improved by lead had to gradually withdraw from the stage of history under the constraint of environmental protection laws and regulations.
Either from aspects of environmental laws and regulations all over the world or technical or economic aspects, there is no value to improve lead brass. The only way is to develop new lead-free copper alloys. The researches on metals, alloys and compounds are a long-term accumulation process, and knowledge about their characteristics is very rich for us now. It is a consensus that adding Bi, Sb, Mg, P, Si, S, Ca, Te, Se, etc to copper alloy could improve its cutting performance, while many related patents all over the world were published. It must be pointed out that compared to easy-cutting lead brass, all the easy-cutting lead-free copper alloys at present have higher cost or/and lower processing performance or/and application, such as cold and hot workability, cutting performance, anti-dezincification resistance, ammonia resistance, etc. The comprehensive performance and cost performance of lead-free copper alloy are much worse than that of lead brass. Bismuth could be used to improve the cutting performance of copper alloys, but the copper alloy with high mass fraction of bismuth is unacceptable by the market due to its high price. The copper alloys with low mass fraction of bismuth have good cutting performance, but there is still a big gap compared with lead brass. On the other hand, it is not clear so far about the effect of bismuth ion on human health, and its side effects are inconclusive, so bismuth brass is not accepted in some countries and regions. It is also doomed that bismuth could not be used as the main alternative element of lead in easy-cutting lead-copper alloys because of its limited resources. The copper alloy would have tendency of brittleness after bismuth being added, and deteriorate the pressure processing performance seriously, especially hot work performance. The recycled bismuth containing copper alloy would harm the copper processing industry, seriously decrease its value of recycling, which is unfavorable for the market promotion of easy-cutting bismuth containing copper alloys. Antimony is an element which is minimally toxic to human body. Its leaching concentration in water is severely restricted. The use of antimony brass is restricted although it has good cutting performance. It is unfavorable for the market promotion because of the less desirable hot work performance of antimony brass and the high price of antimony. Magnesium can improve cutting performance of brass obviously, but its mass fraction cannot be excessive. The elongation of Mg-brass would decrease when the mass fraction of magnesium being larger than 0.2 wt. %, and the decreasing rate will rapidly increase when increasing the mass fraction of the magnesium, which is unfavorable for the application of Mg-brass. Magnesium is a big burnt-loss element, which is a big challenge for the mass fraction control of magnesium Mg-brass. The cutting performance of brass would be improved after phosphorus being added, but its plasticity would decrease, and its tendency of hot crack would increase when being cast under low pressure. So, it would severely restrict the adding amount of phosphorus and the application of phosphorus brass. Because of high price of tin, tellurium and selenium, brass containing the tin, tellurium and selenium are difficult to be promoted widely in the market. Tin can barely improve the cutting performance of copper alloys. There are two silicon brasses among published patents. One is low-Zn silicon brass, such as C69300, which has a small market share due to its high mass fraction of copper, high density and high price. Another is the high-Zn silicon brass which has low cutting performance. Sulfur would easily pollute the surroundings when being added into the copper alloys due to its low melting point (113° C.) and low boiling point (445° C.), which cannot meet the requirements of free-pollution in today's increasingly stringent environmental regulations. Therefore, it is also extremely unfavorable for its marketing and application. In the copper alloy which does not contain manganese, sulfur usually presents in the grain boundaries in the form of low melting point eutectic. So, the copper alloy is hot brittle, and it is difficult for easy-cutting sulfur copper alloy to be hot wrought. Besides, its cost is relatively high. If sulfur or sulfides that have an affinity to sulphur less than the affinity of manganese to sulphur was added into brass fused mass, the sulfur or sulfides would react with manganese in brass fused mass and produce manganese sulfide which would float out as slag in the copper alloy fused mass, decreasing and even obliterating the cutting performance of sulfur. The mass fraction of Zn in brass is high. Zn is a typical volatile metal, and the manganese sulfide which is a product of reaction between manganese and sulfur in the brass fused mass is easily to be brought to the surface of the fused mass by gaseous Zn. Moreover, spitfire technology are usually used to degas the brass after being taken out of the furnace, which leads the reacted manganese sulfide slag to be brought to the surface of the fused mass and removed as the form of slag. This is one of the important reasons why manganese and sulfur are difficult to coexistent in cast brass. The Chinese patent 201110035313.7 indicated that the small ingot has good cutting performance in laboratory, but the requirements which is mentioned according to claim 3: “adding Zn quickly, then immediately casting into ingots” cannot be met in the industrial massive manufacturing. The cutting performance of manganese sulfide product decreases rapidly with the duration of copper alloys fused mass in the furnace, even disappears at last. Furthermore, with increase of the mass fraction of the sulfur, manganese sulfide product increases and the corresponding slag float faster, and its cutting performance decreases more rapidly. According to the easy-cutting mechanism for copper-manganese alloy sulfide, under the condition without significantly deterioration of the process and application of copper alloy, the higher the mass fraction of sulfur is, the more the manganese sulfides are produced, and the better the cutting performance of copper alloy is. But when copper alloys were cast, the manganese sulfides float from the fused mass more easily, and its effect of improving cutting performance decreases more rapidly. It could be concluded that high-sulfur copper-manganese alloy could not be produced by casting. The method of multi-element alloy was used mostly to improve the cutting performance of copper alloys, for example, the combinative elements were added into copper alloys. But in practice, it is proved that adding many elements which could improve the cutting performance is not an ideal way. On one hand, the interaction between the elements could decrease the cutting performance of copper alloys. On the other hand, the copper alloy would be strengthened by combinative elements adding, which would increase the strength and hardness of the copper alloy, and decrease the performances of pressure processing and the machine work of copper alloys. Besides, adding too many rare and expensive elements would increase the cost of copper alloys, which is also unfavorable for its marketing and application. There are still limitations in adding combinative elements to improve processing and application of copper alloys.
The lead-copper alloys were often used as self-lubricating bearing which contain oil, but they doomed to be replaced. Graphite is also added to the copper alloy because graphite has excellent lubricating ability and it is one of the widely used lubricants. Just like lead, graphite is hardly solid-soluble in copper, and its interface with copper is mechanical engagement rather than metallurgical bonding, resulting in low interfacial strength, which results in low strength of graphite self-lubricating bearings, and it cannot meet the requirements in heavy-duty and high-speed environment.
There is an urgent need for a new lead-free, easy-cutting copper alloy, which not only have excellent processing such as cutting performance, hot forging, polishing and plating, but also have excellent application such as high strength, anti-dezincification resistance, ammonia resistance and self-lubricating properties. The present invention has been developed under these considerations.